1. Field of the Invention
The present invention relates to an electronic scanning radar apparatus preferably equipped in a moving body, a receiving wave direction estimating method, and a computer-readable storage media storing a receiving wave direction estimation program in which the electronic scanning radar apparatus transmits a transmission wave to a target so as to detect the target by receiving a reflection wave of the transmission wave reflected from the target.
2. Description of the Related Art
In general, an electronic scanning radar apparatus is known as an on-board radar. For the radar, a frequency modulated continuous wave (FMCW) radar, a multiple-frequency continuous wave radar, a pulse radar or the like is employed.
For each radar described above, a receiving wave direction estimating method with array antennas is used for detecting the direction of a receiving wave from a target. The receiving wave may be referred to as an incoming wave, and the target may be referred to as a reflecting object.
Recently, the receiving wave direction estimating method employs highly accurate algorithms such as an auto regressive spectral estimation method (AR spectral estimation method; see FIG. 33), a multiple signal classification method (MUSIC method) or the like, which can provide high resolution (high accuracy) of the direction of receiving waves without increasing channels of receiving antennas. These methods are described in Japanese Unexamined Patent Application, First Publication, Nos. 2006-275840, 2007-40806 and 2009-156582. These methods are also described in “MATLAB Multi-media Signal Processing part I: Digital signal fundamentals” published by Ikehara and Shimamura in 2004 by BAIFUKAN CO., LTD (hereafter referred to as nonpatent publication 1), and described in “Adaptive Signal Processing with Array Antennas” published by Kikuma in 1998 by Kagaku Shuppan Co. LTD (hereafter referred to as nonpatent publication 2).
As the AR spectral estimation method a maximum entropy method (MEM) or a linear prediction method can be used.
For estimating the direction of receiving waves from a target (reflecting object) with those algorithms, input data indicated by complex numbers are converted into a matrix form called as a correlation matrix, and then the estimation process is performed. The input data is formed by synthesizing a complex sine wave and a noise component.
Japanese Unexamined Patent Application, First Publication, Nos. 2007-40806 and 2009-156582 described a receiving wave direction estimation process of radar mounted in a car, in which the direction estimation correlation matrixes in past detecting cycles are stored in a memory, and then the correlation matrixes are averaged (or performed by an addition process) with a correlation matrix in the present detecting cycle in order to reduce noise components. This averaging process of the correlation matrixes can reduce noise factors (components), so that the accuracy of the receiving wave direction estimation is improved.
For the fewer the number of channels of the array antennas, a radar apparatus can be manufactured at less cost with smaller size, which is suitable for an on-vehicle radar apparatus. On the other hand, the accuracy of estimation becomes lower even if a highly accurate algorithm is used, when the amount of data sets (data) becomes insufficient for an accurate estimation. Namely, when the amount of data sets becomes insufficient, it becomes difficult to treat the receiving signals as ideal sine waves. This influences the results of correlation processes. For this problem, the averaging process is performed between the correlation matrixes, so that the accuracy of the detection of a target becomes greatly improved.
Japanese Unexamined Patent Application, First Publication, No. 2007-40806, describes that the correlation matrixes must be stored in the memory (or storage device) as information that corresponds to the whole distance (frequency) points. This requires a mass storage device (or large capacity memory) when the detection range is expanded or the detection accuracy is improved. The past correlation matrixes have beat frequencies that correspond to those of the present correlation matrix. This condition provides proper averages for detecting the direction of the target as long as the electronic scanning radar apparatus follows the target in a constant distance. However, when the distance between the electronic scanning radar apparatus and the target is varied, there is a possibility that the beat frequency of the present correlation matrix does not correspond to those of the past correlation matrixes. This could degrade the data used for detecting the direction of the target.
It is described in Japanese Unexamined Patent Application, First Publication, No. 2009-156582 that the storage capacity of a memory can be reduced, compared to the case of Japanese Unexamined Patent Application, First Publication, No. 2007-40806, by only storing distant points (frequency points) of targets in the memory, in which the targets have been determined. However, in order to improve the estimation accuracy, the amount of data (correlation matrixes) used in the averaging process needs to be increased for performing the averaging process. In order to increase the amount of data to be averaged, either the amount of storing of past data (correlation matrixes) stored or the amount of data to be used in an identical detection cycle needs to be increased. For example, if the amount of data to be used in the identical detection cycle is increased when using the FMCW millimeter wave radar, the number of triangular waves needs be increased. In such a case, a challenge will be to solve issues concerning an increase in the capacity of memories and an increase in computing load. For example, the increase in the computing load is caused by increases in the amount of FFT operation which processes along time axis data for every channels, the amount of operations of combination processes of DBF (Digital Beam Forming), and operations of peak detecting processes.